1. Field of the Invention
The present invention generally relates to automatic gain control in a receiver, and more specifically to a programmable gain amplifier (PGA) that performs automatic gain control while minimizing transient voltages during tap changes.
2. Background Art
In electronic communications, electromagnetic signals carry information between two nodes over a connecting medium. Exemplary media include cable, optical fiber, public airways, etc. The signal strength at the receiving node varies depending on the distance between the nodes and changes in the condition of the medium. For example, the signal strength typically decreases with increasing distance between the two nodes. Furthermore, even if the distance is fixed, physical variations in the medium over time can affect signal strength. For example, in a cable system, different cables can have different attenuation constants. Also, increased moisture content in a cable line, or in the public airways can reduce signal strength at the receiver. Finally, variations in transmitter output power will also affect signal strength at the receiver.
An automatic gain control (AGC) circuit and a programmable gain amplifier (PGA) are often used at the receiver input to compensate for variations of received signal strength. More specifically, the AGC circuit adjusts the gain setting of the PGA to maintain the signal strength within a desired operating range. If the received signal strength is too high, then the AGC lowers the gain setting of the PGA. If the received signal strength is too low, then the AGC raises the gain setting of the PGA. When the AGC is changing the gain of the PGA, there is a possibility of introducing a glitch in the system. The glitch manifests itself as an unwanted transient voltage that can cause a voltage detection error if the transient voltage does not settle within specified time period, for example one clock cycle.
What is needed is PGA configuration that quickly settles any transient voltage caused by changing gain settings. Furthermore, the PGA configuration should have sufficient operating bandwidth.
The present invention is a programable gain amplifier (PGA) having an amplifier and a variable resistor that is connected to the output of the amplifier. The variable resistor includes a resistor that is connected to a ground or reference voltage, and multiple parallel taps that tap off the resistor. Additionally, the PGA includes a two-stage switch network having fine stage switches and coarse stage switches that connect the resistor taps to an output node of the PGA. The taps and corresponding fine stage switches are arranged into two or more groups, where each group has n-fine stage switches and corresponding taps. One terminal of each fine stage switch is connected to the corresponding resistor tap, and the other terminal is connected to an output terminal for the corresponding group. The coarse stage switches are connected to corresponding group output terminals and select a group of fine stage switches to connect to the output of the PGA.
During operation, one tap is selected to be connected to the output of the PGA by closing the appropriate fine stage switch and coarse stage switch, where the selected tap defines a selected group of the fine stage switches. Additionally, one fine stage switch is closed in each of the non-selected groups of fine stage switches. In one embodiment, the location of the closed switches in the non-selected groups is the mirror image of the location in an adjacent group. In other words, if the mth fine stage switch is closed in a first group of fine stage switches, then the [(n+1)xe2x88x92m]th fine stage switch is closed a second group of fine stage switches that is adjacent to the first group of fine stage switches, assuming the fine stage switches are indexed from 1-to-n in each group. This reduces the transient voltages that occur when tap selection changes from one group to another.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.